Simulcasting is the practice of broadcasting a single radio-frequency signal from multiple locations at the same time. Simulcast transmission networks are established when it is desirable to maximize the area over which receivers tuned to the network's broadcast frequency are able to pick up and process the broadcast signal. A paging system is one type of radio system that is operated as a simulcast system. In a paging system, system subscribers are provided with small radio receivers, called pagers. The paging system further includes one or more paging terminals and a number of transmitter sites. The paging terminals are connected to the publicly switched telephone network and receive calls for the individual system subscribers. In response to receiving an incoming call, a paging terminal will generate a message, a page. The page is forwarded to the transmitter sites, which broadcast the page for receipt by the subscriber's pager. When a page is received by a pager for the subscriber to whom the pager is assigned, an annunciator or display integral with the pager is actuated to inform the subscriber of the call. Paging systems are provided with multiple, spaced-apart transmitter sites to maximize the coverage area in which a pager can function.
The individual transmitter sites of a paging system and of other simulcast networks must operate in concert so as to transmit the same signal at the exact same instant. This is important because pagers or other receivers in areas where signals from two or more transmitting sites can be received will receive signals from each transmitter site. If the signals are out of phase, their sum produces a single signal that frequently cannot be processed by the receiver. Thus, paging systems and like broadcast systems are typically constructed so that each transmitter site broadcasts the same signal simultaneously. This ensures that in overlap areas the signals from multiple transmitter sites will be in phase and combine to produce a single signal that can readily be processed by the intended receiver.
Many simulcast systems have some type of analog signal link network between the central station, from which the signal to be broadcast originates, and the individual transmitter sites from which the signal is finally broadcast. A link network may take the form of a telephone or fiber-optic signal link between the central station and one or more particular transmitter sites. A link may alternatively take the form of a radio link between the central station and the transmitter site over a carrier frequency different from that over which the transmitter sites actually broadcast the simulcast signals The links to some transmitter sites may actually be a multi-link connection. For example, it is not uncommon for a central station to first forward the signal to be simulcast to a satellite transmitter. The signal is sent up to a satellite, which retransmits it to one or more transmitter sites. In some of these systems each transmitter site includes a delay circuit that regulates when the received signals are to be rebroadcast. Collectively, the individual delay circuits are set to ensure that the transmitter sites associated therewith all broadcast the same signal at the same time.
While analog systems have proved useful for simulcast applications, they are not without disadvantages. Some analog simulcast systems require significant amounts of air time to constantly send new delay rate instructions to the individual transmitter sites to ensure that they all transmit the same signal. This "overhead" air time may significantly depreciate the amount of air time that is available to forward signals containing useful data, such as paging signals, to the transmitter sites for rebroadcast. Furthermore, the actual transmission delay time for any transmit station is a function of the link propagation time, the time it takes the signals to travel from the central station to the transmit station. If this link should change, as may happen because of an intermediate component failure, or due to a change in the day-to-day economics of running particular links, then the link propagation time will change. For example, the satellite receiver at one transmitter station may be taken out of service for maintenance; in order to maintain the transmission of simulcast data to the site, a link over the publicly switched telephone network may be temporarily established. Until a new link propagation time is forwarded to a transmitter site, that site will broadcast the page at a time out of phase with those broadcast by the surrounding sites. Still another disadvantage of many simulcast transmission systems is that they require maintenance receivers to continually monitor the delay between the time a signal is first forwarded by a central unit and the time it is finally broadcast by a transmitter site; this information is then used by the system's control circuitry to regularly adjust the transmission delay times for the individual transmitting sites.
A further limitation associated with analog linking networks is that in each step of the central station-to-transmitter site signal transfer, there is degradation of the signal. This happens as a result of the normal signal loss that occurs when an analog signal is processed by an amplifier and that occurs as a consequence of the broadcast and transmission of a radio signal. Usually, the longer the link between the central station and the transmitter site and/or the more times the signal is subjected to intermediate processing, the greater the degradation from the original signal. As a consequence of this degradation, the final signal that is received by the transmitter site for simultaneous broadcast may be so changed that the actual broadcast signal is unintelligible by the receivers for which it was intended.